The prime object of this project has become the detection of serious behavioral deterioration in rhesus infected with SIV. Such deterioration may indicate a simian condition homologous to the so-called AIDS related dementia that occurs in a proportion of humans infected with HIV, usually in the late stages of the disease. Late stage SIV infection is not in itself sufficient to lead to behavioral deterioration until death is imminent, and does not do so in most subjects. Nine subjects, infected with SIV, have now been studied longitudinally each for many months, working daily on a full work schedule of 5 components. The performances of the subjects were similar and consistent. Five have now died of SIV infection. Three showed no deterioration in their usual behavioral performance until the last days before death. Two monkeys have shown profound behavioral deficits before physical deterioration for weeks before death. The changes were similar for the 2 animals. Subjects would work normally under 1 or 2 of the components and not at all under other components. This pattern is unprecedented in my experience. If the phenomenon is AIDS related dementia in the rhesus, intensive chemico-pathological studies on the brains of simian subjects showing the syndrome could be performed, included procedures not feasible in humans, providing at last an opportunity to make progress in elucidating pathogenesis.